Hi,
I’ve noticed this strange behavior that if I line up UI separators of editor precisely, they start to move together. Is this a bug or a feature? And if the latter is the case, how do I toggle it off? I am struggling to find a way to unlink them without having to close one of the editor panels and re-create it from scratch… 
As far as I know that’s just the way it works now, and like you say, the way to workaround this is to make a new editor area.
An idea I had to fix this is if you hold some modifier key and drag the corner it could skip moving the aligned edges.
Yes, but this must have been some recently introduced change, as I remember aligning my editors precisely because of my OCD but it never got stuck like this before 
It works as in 2.79, too.
I think it always did.
With 2.8 UI, separators are thicker.
That is probably making the alignment easier and the behavior more noticeable.
To avoid that, I would simply create an offset by creating a new area and deleting it, joining it with desired one.
Corners are already making a lot of different stuff using modifiers (duplicate area as new window, swap area).
As the main goal is to move the separator, I suggest to use modifier when mouse pointer is pressed over line of separator.
Yeah, I think before you could get them very close and they wouldn’t align, now they snap a bit more.
Right, I wasn’t thinking there, I meant the edge rather than the corner.
Yeah, that’s probably it. Can’t believe i never encountered it before